This invention relates to light meter arrangements for TTL (Through-the-Lens) type single lens reflex cameras. Various light meter arrangements have been proposed for the so-called "TTL" photometry systems. These conventional light meter arrangements may be classified into two main groups with respect to the position of the photo-cell. In one of which the arrangement is with a photo-cell positioned between the photographing lens and the focusing screen, and the arrangement in the other group with a photo-cell positioned between the focusing screen and the eye-piece. The invention relates more particularly to light meter arrangements in the latter group.
The prior art light meter arrangements in the latter group may be summarized as follows which will be made with reference to FIGS. 1 and 2. FIG. 1 schematically illustrates an arrangement of the basic components of a view finder for a single lens reflex camera. A tiltable mirror M which is located between a photographing lens not shown and a focal plane shutter not shown, in its reflecting position, reflects light entering through the photographing lens upward onto a focusing screen 1, where an image of the object area intended to be photographed is formed. Light from the image on the screen 1 is radiated upward through a condensor lens 2 into a pentagonal roof prism (hereinafter referred to as "pentaprism"). In the pentaprism it is reflected from one of first two reflecting surfaces 3a and 3a' constituting the roof portion of the pentaprism to a second reflecting surface or front reflecting surface 3b, and directed therefrom through an exit surface 3c to an eyepiece 4. In this case, instead of using the pentaprism, it is possible to use a hollowed reflection complex having three mirrors located at positions corresponding to those of the roof and front reflecting surfaces. A character L indicates a beam of light rays entering the mirror M through the photographing lens and going upward to the eyepiece along the axis of the view finder optic.
FIG. 2 illustrates positions of photo-cells P.sub.1 -P.sub.8 disposed between the focusing screen 1 and the eyepiece 4 and paths of light beams impinge on the photo-cells.
(1) Arrangement with photo-cell P.sub.1 requires a partly mirrored surface provided in the condenser lens 2 to deflect part of the light radiated from the entire image area to the photo-cell P.sub.1.
(2) Arrangement with photo-cell P.sub.2 or P.sub.3 requires a partly light-permeable surface provided on the first reflecting surfaces of the pentaprism at a portion to permit light rays outside of the finder light beam, or part of the light rays going to the eyepiece to reach the photo-cell P.sub.2 or P.sub.3.
(3) Arrangement with photo-cell P.sub.4 requires a partly light-permeable surface provided on the second reflecting surface of the pentaprism at a portion to permit part of the light rays going to the eyepiece to reach the photo-cell P.sub.4.
(4) Arrangement with photo-cell P.sub.5 requires a partly mirrored surface provided in a beam splitter positioned between the exit surface of the pentaprism and the eyepiece to deflect part of the light rays going to the eyepiece to the photo-cell P.sub.5.
(5) Arrangement with photo-cells P.sub.6 and P.sub.6 ' requires for the photo-cells to be positioned behind the exit surface 3c of the pentaprism on the left and right hand sides of the beam entering the eyepiece within the light rays going toward the eyepiece but outside of the beam entering the eyepiece.
(6) Arrangement with photo-cell P.sub.7 requires for the photo-cell to be positioned within the light rays going toward the eyepiece but outside of the light beam entering the eyepiece to receive a beam of light rays from an upper portion of the entire image area on the focusing screen, or a portion on the screen nearer to the lens barrel.
(7) Arrangement with photo-cell P.sub.8 requires for the photo-cell to be positioned within the light rays going toward the eyepiece but outside of the light beam entering the eyepiece to receive a beam of light rays from a lower portion of the entire image area on the focusing screen, or a portion on the screen farther from the lens barrel.
Of these, arrangements (1), (2), (3), and (4), all of which employ means for deflecting part of the light beam entering the eyepiece to the photo-cell are advantageous for facilitating the photometric measurement for the light from the central portion of the entire image area and for rendering the photometric measurement reliable, because the intensity of that portion of the light radiated from the entire image area on the focusing screen which is effectively directed to the eyepiece can be measured to produce an output in good proportion to the light intensity as sensed by the photo-cell regardless of variation of the diaphragm aperture and of employment of a different interchangeable objective having different largest aperture on focal lengths of lenses. However, the brightness of the finder image is reduced at a portion, or as a whole by an amount corresponding to the deflected part of the light rays which would otherwise enter the eyepiece, thereby it being made more difficult to achieve accurate focusing operation.
The other arrangements (5), (6) and (7) are free from the above-mentioned disadvantage of providing finder images of reduced brightness due to the incorporation of the photo-cell as well as the disadvantage of producing a shadow of the photo-cell in the field of view of the finder. However, the photo-cell has to be arranged in spaced apart relation to the axis of the finder optic in a distance such that it is impossible to measure the intensity of light from the central portion of the entire image area on the focusing screen. In other words, the photo-cell P.sub.6 and P.sub.6 ' in arrangement (5) is responsive to the light beams from portions of the image on the screen which are laterally deviated from the central portion thereof. The photo-cell P.sub.7 in arrangement (6) is responsive to the light beam coming from an upper portion of the view field in the object space. The photo-cell P.sub.8 in arrangement (7) is responsive to the light beam coming from a lower portion of the view field in the object space. Therefore, it has been conventionally practiced to select a plurality of photo-cells for employment in a light meter arrangement and employ an electric circuit adapted for combination of the outputs of all the photo-cells to derive an output which is in effect substantially equal to that which might have been obtained by a photo-cell arranged to receive the light from the central portion of the entire image area on the focusing screen.